<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<style>
  div {
    width: 100px;
    height: 100px;
    background-color: red;
  }

  .box1 {
    background-color: blue;
    width: 200px;
    height: 200px;
  }
</style>

<body>
  <div></div>
</body>
<script>
  const box = document.querySelector('div');
  box.className = 'box1';
</script>

</html>